Cell proliferation may be controlled by secreted growth factors which act as either positive or negative regulators of cell growth, either locally or at a distance. Phenotypic transformation of normal cells may be achieved by constitutive activators of growth factor signaling pathways. Similarly, loss of normal cell growth inhibitory signal transduction pathways can also contribute to neoplasmic transformation. Growth inhibitory factors, termed TIFs, were first identified in the conditional media of A673 cells: TIF activities were separated into two distinct components TIF-1 and TIF-2. TIF-2 was identified as Interleukin-7 alpha. TIF-1 has not previously been characterized. We thus studied its biological activity and characterized TIF-1 by MALDI-MS and sequence analysis. A paper describing our findings is in preparation.